[Smokin' Joe]

Perhaps, as suggested before, the explosion was not meant to kill people - immediately, but carried something more frightening. Biological? Something that in the coming days will prove deadly?

One possibility, which cannot be discounted yet...

It is also possible that the intended explosives were made at the same time (same batch) by the terrorists and they screwed the pooch on the mixing job. If so, they should be good information sources, provided they can be apprehended.

Someone might have sold them bad or bogus explosives, too, with just enough to smell right, but not much bang for the buck.

Regardless, they have disrupted the transit system, even if they have not been generally effective in the murder part of their murder and mayhem campaign.

[stockpirate]

Appears that it was a full scale attack that failed because the bombs failed to detonate.